The present invention relates to a flow sensor and more particularly to one which is suitable for sensing a very small flow rate of gas. Conventionally, a flow sensor for the measurement of small flow rates of gas has a construction that includes a substrate with a through hole or a cavity formed therein; has a heating portion and a sensing portion, both of which are formed in a thin layer bridged over the through hole or the cavity and supported at both ends or at one end only.
Heat generated from the heating layer is transferred to the sensing layer in an amount corresponding to the flow rate of gas to be determined by the sensed heat's value.
To effectively heat the sensing portion, it is necessary to form the heating portion and the sensing portion possibly close to each other on the substrate by using micromachining technology. The known art however, minimized the distance between the heating portion and the sensing portion by only 2.about.3 .mu.m. In addition, the known art had its limits in improving the heat's transfer efficiency because the sensing portion had one end near the heating portion but the other end far from it.
Accordingly, the transfer of heat from the heating portion to the sensing portion was uneven, i.e., efficient at the nearest end and inefficient at the furthest end.
During the measurement of the gas flow, the nearer end of the sensing portion may be, too overheated to fuse. Fusing may also occur in the heating portion when its downstream side is heated more than the upstream side by the gas flowing along that portion during the time of measuring. This can be summarized thus, that the known art sensor caused a deviation in the distribution of heat in the direction of the gas flow to be sensed.
A variety of attempts to improve the detection limits of the flow sensor in relation to its heating and sensing portions have been proposed. For example, in order to cancel the difference in output voltage, as well as the error factor and the difficulties in signal processing in a discriminator circuit, attempts were made to increase the output voltage by increasing the resistance values of the heating and the sensing elements.
The resistance values of the heating and sensing elements can be increased, e.g, by thinning the film of resistance or by narrowing a pattern's width or by increasing a pattern's length. However, thinning the resistance film may cause an increase in the defective lot and the impairment of the durability and aging characteristics. Many resistance films of less than 0.3 .mu.m in thickness may be defective. Narrowing the pattern by micro-machining is limited to 1 .mu.m. Further narrowing causes problems with unevenness in size and quality, in production yield and in aging. Increasing the pattern's length increases the pattern's area and may increase power consumption and extend the time response. In the case of a large pattern, the distance from the heating element's end to the sensing element's end increases making it impossible to obtain the effective transfer of heat therebetween.